1. Field of the Invention
The present invention relates to a test circuit for judging a threshold of a signal input circuit which inputs an analog or digital signal to converts it to a binary signal at a predetermined threshold. The binary signal is supplied to various logic circuits, such as registers, microcomputers, and the like.
2. Description of Related Art
FIG. 1 is a block diagram showing a configuration example of a test circuit of a conventional signal input circuit (hereinafter, to be called an input threshold circuit) having a threshold. In this conventional example, a circuit testing a threshold of an operational amplifier as the input threshold circuit, to which am analog signal is inputted, is shown as an example.
In FIG. 1, reference numeral 1 designates an analog input terminal, 2 a reference voltage input terminal, 3 an operational amplifier as the input threshold circuit, 4 an operational amplifier output signal, 5 a sampling clock, 6 a sampling circuit, 7 an output signal (hereinafter, to be called a sampling output) of the sampling circuit 6, 8 a register as a logic circuit, 10 an output dedicated line, 12 an output circuit, and 13 designates a judging terminal.
Next, explanation will be given on the operation.
In the circuit shown in FIG. 1, a certain constant voltage, 1 V, for example, is inputted as a threshold to the reference voltage input terminal 2, and voltage more than the reference voltage, 1.5 V, for example is inputted to the analog input terminal 1. Thereby, the operational amplifier 3 operates responsive to voltages of the input signals form the two input terminals and outputs "1" as the output signal
On the contrary, when voltage less than the reference voltage, 0.5 V, for example is inputted to the analog input terminal 1, the operational amplifier 3 outputs "0" as the output signal 4.
By the fact that the sampling circuit 6 takes in the output signal 4 of the operational amplifier 3 in synchronism with the sampling clock 5, the sampling output 7 is obtained. The sampling clock 5 is supplied from the outside, or the internal system clock or the frequency dividing clock thereof is used as the sampling clock.
Next, the sampling output 7 is temporarily stored in the register 8 with software processing, then outputted to the output dedicated line 10 with software processing. Since the output signal from the register 8 is outputted to the judging terminal 13 through the output circuit 12, when the voltage of the analog signal inputted to the analog input terminal 1 of the operational amplifier 3 is changed step by step, the actual threshold of the operational amplifier 3 becomes clear by reading the value of the output signal. In such a way, by the circuit shown in FIG. 1, it is possible to test whether or not the operational amplifier 3 is operated normally.
By the way, since the conventional test circuit for the signal input circuit having a threshold shown in FIG. 1 performs inputting of the sampling output 7 to the register 8 and outputting thereof from the register 8 with the software, time is required for the setting, therefore testing efficiency is low. Especially, in the example shown in FIG. 1, since the output signal of the operational amplifier 3 is outputted to the output circuit 12 after it is temporarily stored in the register 8, the value of the output signal of the operational amplifier 3 is outputted intact to the output circuit 12, but in the case where a microcomputer, for example, is connected as a logic circuit instead of the register 8, a testing program for adjusting the relationship between the value of the output signal of the operational amplifier 3 and the value of the signal outputted to the output circuit 12 is required.
Recently, to say nothing of a microcomputer, the voltage used in the other kinds of logic circuits has been made low. Accordingly, improvement of test accuracy of the signal input circuit for inputting signal to various kind of logic circuits, including a microcomputer, is desired. This is because there is a necessity to test a threshold of the signal input circuit more accurately compared with the conventional one, since a difference between the maximum voltage of signal inputted to the signal input circuit and the minimum voltage thereof becomes smaller as the voltage of the logic circuit is made lower.
Moreover, conventionally, a test circuit such as mentioned above is required for various kind of respective logic circuits of an integrated circuit configured on one chip. Therefore, when test circuit such as mentioned above are provided for the various kinds of respective logic circuits of the integrated circuit, the test circuits occupy a large area on a chip, causing a bad effect upon mounting efficiency.